Cover ring and shield supporting a wafer ring in a plasma reactor

ABSTRACT

A magnetic dipole ring assembly positioned inside a vacuum chamber and around a wafer being sputter deposited with a ferromagnetic material such as NiFe or other magnetic materials so that the material is deposited with a predetermined magnetization direction in the plane of the wafer. The magnetic dipole ring may include 8 or more arc-shaped magnet segments arranged in a circle with the respective magnetization directions precessing by 720° around the ring. The dipole ring is preferably encapsulated in a vacuum-tight stainless steel carrier and placed inside the vacuum chamber. The carrier may be detachably mounted on a cover ring, on the shield, or on the interior of the chamber sidewall. In another embodiment, the magnet is a magnetic disk placed under the wafer. Such auxiliary magnets allow the magnetron sputter deposition of aligned magnetic layers.

RELATED APPLICATION

[0001] This application is a division of Ser. No. 10/068,669, filed Feb.5, 2002 and to be issued as U.S. Pat. No. 6,743,340.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The invention relates generally to sputtering of materials. Inparticular, the invention relates to the auxiliary magnets used in thesputtering of magnetic materials.

[0004] 2. Background Art

[0005] Magnetic random access magnetic memory (MRAM), also calledmagnetoelectronic memory, is receiving increased interest and isexpected to shortly enter commercial manufacture. MRAM as currentlyconceived involves the integration of magnetic materials intosemiconductor integrated circuits to produce chips having millions ofmemory cells on which information can be written and read. When it isfabricated on a silicon wafer, silicon support circuitry can be includedon the same wafer. Importantly, the MRAM is non-volatile memory. In awrite operation, the magnetic material is poled into one of two magneticstates. In a read operation, the magnetic state of the poled material isdetermined. The magnetic state of the memory is maintained in thequiescent period between the write and read operations with no powerbeing applied to the memory cell.

[0006] Many forms of MRAM have been contemplated, some of which arereviewed by Johnson in “Magnetoelectronic memories last and last . . .,” IEEE Spectrum, vol. 37, no. 2, February 2000, pp. 33-40. One formincludes a magnetic tunneling junction (MTJ), which is explained in moredetail by Parkin et al. in “Exchange-biased magnetic tunnel junctionsand application to nonvolatile magnetic random access memory,” Journalof Applied Physics, vol. 85, no. 8, 15 Apr. 1999, pp. 5828-5833. FIG. 1is a schematic orthographic view of one MTJ cell 10 in a largetwo-dimensional array. The many cells 10 are formed by fairly standardtechniques well developed for the most part in the semiconductorintegrated circuit industry. Furthermore, when the magnetic cell isfabricated on a silicon wafer, silicon support circuitry can beintegrated on the same wafer as the magnetic memory.

[0007] The magnetic storage cell 10 is centered on a junction structure12 including a fixed magnetic layer 14 and a free magnetic layer 16separated by a very thin non-magnetic tunneling barrier 18. Bothmagnetic layer 14, 16 are relatively thin, typically on the order of 1to 20 nm thick. In the most prevalent MRAM design, the magnetic layers14, 18 are electrically conductive, and the tunneling barrier 18 is avery thin electrically insulating layer, typically on the order of lessthan 2 mn or even 1 nm. The extreme thinness allows quantum mechanicalelectron tunneling through the otherwise dielectric barrier 18. Analternative structure replaces the dielectric barrier with a metalbarrier through which spin can tunnel, as described by Tehrani et al. in“High density submicron magnetoresistive random access memory,” Journalof Applied Physics, vol. 85, no. 8, 15 Apr. 1999, pp. 5822-5827.Somewhat similar magnetic stacks may be used to form spin valves or spintransistors, as Zorbette describes in “The quest for the spintransistor,” IEEE Spectrum, vol. 38, no. 12, December 2001, pp. 30-35.

[0008] The two magnetic layers 14, 16 of the MTJ cell 10 aredistinguished in that the fixed magnetic layer 14 has a predeterminedmagnetization, for example, in one of the two horizontal directions ofthe illustrations, while the free magnetic layer 16 can besemi-permanently poled and repoled into either of the two horizontaldirections. Which horizontal direction determines the state of thememory cell 10. The magnetic layers 14, 16 are typical composed oftransition metals and their alloys, for example, NiFe, CoFe, Co, or Ruor a bilayer or sandwich structure of such materials. The iron alloysare typically rich in the transition metal, for example, Ni₈₀Fe₁₀ orCo₉₀Fe₁₀. The barrier 18 may be composed of oxidized aluminum. Thedistinction between fixed and free magnetic layers may be determined bythe fixed layer being grown on an anti-ferromagnetic layer, also calledthe exchange-bias layer which prevents the adjacent fixed magnetic layerfrom changing state. The exchange-bias layer is typically a manganesealloy, for example, Pt₅₀Mn₅₀. Other anti-ferromagnetic compositionsinclude MnFe, MnIr, MnRh, NiO, TbCo, and iridium alloys. Theexchange-bias layer allows the two magnetic layers 14, 16 to have thesame composition. Other buffer, transition, and capping layers aretypically included in the stack structure.

[0009] In the illustrated MTJ cell 10, a metallic bit line 20 iselectrically connected to the free magnetic layer 14 of the storagestructure 12 as well as to many other cells 10 in the plane of theillustration. The fixed magnetic layer 14 is electrically connected to aconductive cross connector 22 electrically selected by an isolationtransistor 24. In this embodiment, it is assumed that the isolationtransistor 24 is individually selected for each memory cell 10. A digitline 26 underlies the storage structure 12 and runs orthogonally to thebit line 18 in the two-dimensional memory array.

[0010] The operation of the memory cell 10 relies upon the effect thatthe impedance of the storage 12 when the two magnetic layers 12, 16 arealigned to be parallel, as illustrated in the schematic illustration ofFIG. 2, is significantly less than the impedance when the two magneticlayer 14, 16 are aligned to be anti-parallel, as illustrated in theschematic illustration of FIG. 3. The impedance depends upon quantummechanical spin effects between the two magnetic layers 14, 16 and maybe measured as either voltage or current by measuring circuitry gated bythe isolation transistor 24, usually in comparison to a reference cell.The measuring electrical path proceeds from the bit line 20 through thestorage structure 12, cross connect 22 and isolation transistor 24.

[0011] It is possible to initially pole the fixed layer 14 (as well asthe free layer 16) with a large current pulse and thereafter inoperation use lesser currents to switch only the free layer 16. However,for a large dense memory, the one-time poling of the fixed layer 14significantly complicates the chip design. It is much preferred that themagnetization direction of the fixed magnetic layer 14 be impressedduring the growth of the fixed layer 14 and that in operation the fixedlayer 14 remain permanently polarized. On the other hand, duringoperation the magnetization direction of the free magnetic layer 16 canbe written and rewritten in selected directions according to orthogonalcurrents passed through the bit line 20 and the digit line 26. Once themagnetic state is written into the storage cell 10, it remains untilrewritten. Whatever state is currently written in the storage cell 10 isread by measuring the impedance of the storage structure 12. It isimportant that the magnetization direction impressed during growth ofthe fixed magnetic layer 14 be properly aligned with the bit and digitlines 20, 26. Any significant misalignment, for example, more than 5 or10°, degrades the impedance differential between the two memory states.

[0012] The magnetic layers 14, 16 are conveniently formed of a magneticmetal such as NiFe or CoFe by a sputtering process using a sputteringtarget of approximately the same composition. Sputtering has theadvantage of a relatively high sputtering rate using relativelyexpensive source materials in a relatively simple sputter reactor.However, sputtering of the stated magnetic layers presents somedifficulties. Usually magnetron sputtering is employed in which amagnetron is positioned in back of the target to create a magnetic fieldin front of the target to increase the plasma density and hence thesputtering rate. However, the magnetic characteristics of the targetinhibits the effect of the magnetron.

[0013] A more difficult problem arises when it is desired to deposit thefixed magnetic layer 14 with its magnetization fixed in a predetermineddirection determined by the bit and digit lines 20, 26. In the past,such selective magnetization during deposition has been accomplished byplacing coils or other means outside the sidewalls of the depositionchamber. The magnetic direction of the magnetic means determines themagnetization of the magnetic metal being deposited. However, theseprior approaches of placing the magnetic means outside the chambersuffer from excessive size and further require a redesign of the vacuumchamber and its ports.

[0014] Sekine et al. in U.S. Pat. No. 5,660,744 have suggested the useof a Halbach magnet array, also called a magnetic dipole ring, forcreating a uniform magnetic field for confining and intensifying aplasma, primarily in the context of reactive ion etching although withbrief reference to sputtering. Such magnet arrays will be described indetail later. However, the Sekine implementation suffers from thedisadvantage of being positioned outside the vacuum chamber and hencerequiring a significant chamber redesign to incorporate a magnet into asputtering reactor system already designed for non-magnetic sputtering.Miyata in U.S. Pat. No. 5,519,373 more explicitly refers to the use of amagnetic dipole ring in a sputter reactor as the principal magnetron forcreating a high density plasma. It too suffers from being locatedoutside the vacuum chamber.

SUMMARY OF THE INVENTION

[0015] Magnetic material and particularly ferromagnetic material may besputter deposited on a substrate by imposing a substantially uniformmagnetic field parallel to the face of the substrate during deposition.The magnet assembly producing the field may be inserted within thevacuum chamber.

[0016] In one embodiment, the magnet assembly is a magnetic dipole ringcomprising a plurality of eight or more magnets arranged in a ringaround the substrate. The magnets may be contiguous segments having arcshapes. The magnetization directions of the magnets precesses by 720°around the ring. Such a dipole magnet ring produces a substantiallyuniform magnetic field inside the ring.

[0017] Advantageously, the dipole ring is encapsulated in a vacuumtight, vacuum compatible material such as stainless steel. Furtheradvantageously, the magnets of the dipole ring are formed of a magneticmaterial having a Curie temperature above 200° C., thereby allowing thesputter reactor to be baked out while the magnet ring is placed insidethe reactor.

[0018] The dipole ring may be detachable mounted on a cover ring used toprotect the periphery of the pedestal supporting the substrate beingsputter coated.

[0019] Alternatively, the dipole ring may be disposed between thechamber shield and the chamber sidewall. In this position, the dipolering may be supported on the chamber sidewall or by the shield.Advantageously, the shield may have a recess accommodating the dipolering.

[0020] In another embodiment, the magnet assembly is a horizontallypolarized magnetic disk placed beneath the substrate being sputtercoated.

[0021] The invention can be applied to ferromagnetic andanti-ferromagnetic materials as well as to other magnetic materialsbenefitting the imposition of magnetic field during deposition.

[0022] The magnet assemblies can be applied also to chemical vapordeposition of magnetic materials.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 is a cross-sectional view of a magnetic tunneling junctionmemory cell.

[0024]FIGS. 2 and 3 are schematic cross-sectional view of the twomagnetic states of the memory cell of FIG. 1.

[0025]FIG. 4 is a schematic cross-sectional view of a sputtering reactoradapted for magnetic sputtering.

[0026]FIG. 5 is a schematic plan view of a magnetic dipole ring usablewith the invention.

[0027]FIG. 6 is an orthographic view of a magnet segment used in thedipole ring of FIG. 5.

[0028]FIGS. 7 and 8 are cross-sectional views of an encapsulated dipolering and its method of fabrication.

[0029]FIG. 9 is an orthographic view of a first embodiment of theinvention including a cover ring configured to support the dipole ringof FIG. 8.

[0030]FIG. 10 is a cross-sectional view of the cover ring of FIG. 9.

[0031]FIG. 11 is a side view of the cutouts formed in the cover ring tosupport the dipole ring.

[0032]FIG. 12 is a plan view of a second embodiment of the invention inwhich the chamber wall supports the dipole ring.

[0033]FIG. 13 is cross-sectional view of a third embodiment of theinvention in which the shield supports the dipole ring.

[0034]FIG. 14 is a plan view of a fourth embodiment of the inventionincluding a horizontal magnetization assembly incorporated into thewafer support pedestal.

[0035]FIG. 15 is an elevational view of the embodiment of FIG. 15.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0036] The invention includes a magnetron sputtering chamberadditionally incorporating a magnet within the chamber to impress ahorizontal magnetic field near the surface of the wafer being sputter orotherwise coated with a magnetic material.

[0037] A specific example of a first embodiment of a magnetron sputterreactor 40 of the invention is illustrated in the schematiccross-sectional view of FIG. 4. The fairly conventional portion of thereactor 40 will be described first. Cha et al. have described details ofsome of the components in U.S. patent application Ser. No. 09/910,585,filed Jul. 20, 2001, now abandoned, published as Publication No.U.S.-2003-0015421-A1, and incorporated herein by reference in itsentirety. The reactor 40 includes a vacuum processing chamberprincipally formed of a chamber body 42 and an adapter 44 formedgenerally symmetrically about a central axis 46 and electricallygrounded. A vacuum pumping system 48 is connected to the chamber througha pumping port 50 and can pump the chamber to a base pressure in therange of 10⁻⁸ Torr. However, an argon gas source 52 supplies argon intothe chamber through a mass flow controller 54 to act as a sputteringworking gas. Typical argon pressures used in sputtering are in the rangeof 0.5 to 5 milliTorr.

[0038] A sputtering target 56 having at least a surface portion composedof the magnetic metal to be sputtered is supported on the adapter 44through an isolator 58. An example target composition is the binarymagnetic alloy Ni₈₀Fe₂₀ although other compositions can enjoy theadvantages of the invention. A bottom shield 60 is supported on theadapter 44 and is electrically grounded to it. A dark space shield 62 issupported on and grounded to the bottom shield 60 and has a shape whichboth protects the isolator 58 from deposition and forms a plasma darkspace in opposition to the side of the target 56. A power supply 66applies a negative DC voltage of approximately −300 to −800 VDC to thetarget 56. The voltage between the target 56 and shield 60 excites theargon into a plasma, and the positive argon ions are attracted to thenegatively biased target 56 to sputter atoms of the target material.

[0039] A wafer 70 to be sputter coated is supported on a pedestal 72 inopposition to the target 56. A bellows 74 vacuum seals the pedestal tothe chamber 42 but allows the pedestal 72 to be vertically movable toallow transfer of the wafer 70 into and out of the chamber. The pedestal72 is however electrically isolated from the chamber 42 and is typicallyleft electrically floating.

[0040] A magnetron 80 is located at the back of the target 56 to createa horizontal magnetic field near the front face of the target 56 to trapelectrons and thereby increase the density of the plasma and increasethe sputtering rate. The magnetron 80 should produce a strong magneticfield in order that it sufficiently penetrate the magnetic target 56 tocreate a reasonably strong magnetic field at the front face of thetarget 56. As illustrated, a nested magnetron of the type described byFu in U.S. Pat. No. 6,290,825 may be used. The nested magnetron 80,which has a relatively small area, includes an inner pole 82 of onevertical magnetic polarity surrounded by an annular outer pole 84 of theopposite polarity surrounding the inner pole 82. A magnetic yoke 86 bothmagnetically couples the two poles 82, 84 and supports them on a motorshaft 88 for rotation about the central axis 46, thereby increasing theuniformity of deposition. Fu's nested magnetron is unbalanced in thatits outer pole 84 has a significantly stronger magnetic intensity thanits inner pole 86, by at least 50%. The magnetic intensity is themagnetic field flux integrated over the surface area of the respectivepole. However, such unbalance is not so crucial in the magneticapplication which may operate at lower power and ionization levels.Other types of magnetrons may be used. The reactor 40 employs twomechanisms to achieve directional sputtering into relatively highaspect-ratio holes. First, the size of the adapter 44 is chosen toproduce long throw sputtering, that is, a relatively large spacingbetween the target 56 and wafer 70, for example a spacing of 190 mm fora 200 mm-diameter wafer 70. A long throw reactor is one in which thespacing between the target and pedestal is at least 75% of the diameterof the wafer. Secondly, a grounded chimney collimator 90 is supported onthe bottom shield 60 about midway between the target 56 and wafer 70 tointercept off-angle sputter particles. The grounded collimator 90 hasthe additional effect of confining the plasma on its side toward thetarget 56 and away from the wafer 70.

[0041] The parts described above are separately fairly conventional innon-magnetic sputtering. The illustrated embodiment of the magneticsputter reactor 40 additionally includes a cover ring 100 sized andconfigured to support a magnetic dipole ring 102 within the plane of thewafer 70. The magnetic dipole ring 102, schematically illustrated in theaxial plan view of FIG. 5, is intended to produce a uniform horizontalmagnetic field B. In its illustrated embodiment, it includes a pluralityof arc-shaped magnetic segments 104 arranged in a circle. As illustratedin the orthographic view of FIG. 6, each segment 104 has a rectangularalmost square cross section about the circumference of the ring 102. Thesegments 104 are permanently magnetized in different directions in theplane of the ring 102 such that when they are assembled in the ringtheir magnetization directions precess by 720° about the circumferenceof the ring 102 where the direction of precession is opposite thedirection of travel around the ring. The effect is akin to a rotatingplanet orbiting around the sun where the length of the planetary day isthe same as the solar year. It is of course understood that the 720°precession is from the first magnet around the circle back to the firstmagnet so that the precession from the first to the last magnet iscorrespondingly less. For the 16-segment embodiment of FIG. 5, themagnetization directions of neighboring segments differ by 45° withrespect to a fixed direction but by 22.5° with respect to the respectiveradii of the ring. The magnetization direction a in fixed coordinates ofthe segment located at the cylindrical angle φ follows the formula$\alpha = {{2\varphi} + {\frac{\pi}{2}.}}$

[0042] Such a magnetic dipole ring 102 is also known as a Halbach magnetring or a magic ring. As described by Sekine et al. in the above citedpatent, it produces in its interior a magnetic field B which issubstantially uniform along a single direction except for substantialedge effects near the periphery of the ring 102 and much reduced edgeeffects closer to the middle. A small number of magnets intensify theedge effects. The direction of the imposed magnetic field is determinedby the magnetization direction of the magnetic dipole, specifically inthe illustrated embodiment, the diameter linking the two segmentsmagnetized in the radial directions of the circle. Using the aboveformula, the direction for which α=0 is the direction of the uniformmagnetic field B and will be referred to as the direction of the dipolering.

[0043] Although the arc-shaped magnet segments are effective atproducing an intense, fairly uniform magnetic field, they are difficultto fabricate. Rectangular or cylindrical magnets may be used to almostthe same effect and with the added advantage of requiring fewerdistinctive magnetization directions so as to reduce the parts list.

[0044] Returning to FIG. 4, the dipole ring 102 creates a substantiallyuniform horizontal magnetic field B extending along a predeterminedhorizontal direction at the exposed face of the wafer 70. When the wafer70 is inserted into the chamber and is laid on the pedestal, itsorientation is carefully controlled such the magnetic field direction ofthe dipole ring 102 coincides with the desired magnetization directionof the ferromagnetic material being sputter coated on the wafer 70, ashas been described with reference to FIGS. 1-3.

[0045] Even though the magnetic sputter reactor uses a magnetrondesigned for creating a high-density plasma and hence capable ofincreasing the ionization fraction of sputtered copper to above 20% inthe case of non-magnetic targets, the magnetic sputter reactor of FIG. 4relies on the strong magnetron to produce a strong magnetic thatpartially passes through the magnetically attenuating target.Additionally, the magnetic sputter reactor is typically operated at arelatively low target power, which further reduces the ionizationfraction. The very thin magnetic layers required in magnetoelectronicsmay be quickly deposited at low target power and plasma density. Therelatively low effective magnetron field inside the sputtering zone andthe low target power result in a low sputter ionization fraction, on theorder of 2% or less. Accordingly, the pedestal 72 may be leftelectrically floating with little effect on the sputter coating. Theabsence of an electric field near the wafer surface eliminatescomplicating E×B effects.

[0046] The design of the dipole ring should take into account severaloperational considerations. The dipole ring is preferably placed withinthe sputter vacuum chamber, which is typically pumped to a baselinevacuum level of 10⁻⁸ Torr or less. Such chamber pressures are achievableby baking the interior of the chamber at about 200° C. during a bake outpumping sequence performed when the chamber is put into operation afterinstallation or maintenance. This interior placement creates twoproblems, loss of magnetization during bake out and vacuumincompatibility of strong magnetic materials.

[0047] The magnetic material should be chosen to have a Curietemperature T_(c) above the bake out temperature. The Curie temperatureis the temperature below which the ferromagnetic material exhibitsspontaneous magnetization. If the poled magnetic material is raisedabove the Curie temperature, its poling direction is lost. That is, whenit is again reduced to below the Curie temperature, its magnetizationdirection is random and the magnetization is likely to form in smallrandomly oriented magnetic domains unless the material is again poled.Some very strong magnetic materials have an insufficient Curietemperature to survive the 200° C. baking. An alloy of samarium cobaltSmCo 27 H is well known and has an acceptable Curie temperature. Thedesignation 27 H refers to the magnetic strength at the maximum productof BH in the magnetization curve. Liu et al. have described even higheroperating temperatures for alloys of FeCuZr with SmCo in “New rare-earthpermanent magnets with an intrinsic coercivity of 10 kOe at 500° C.,”Journal of Applied Physics, vol. 85, no. 8, 15 Apr. 1999, pp. 5660-5662.However, the magnetic materials are not limited to SmCo, and othermagnetic materials such as NdBFe having lower Curie temperatures may beacceptable in some lower temperature applications.

[0048] SmCo magnets like most rare-earth magnets are formed by sinteringmetal particles, in this case particles of samarium and cobalt.Sintering typically involves dispersing the particles in a slurryincluding various sintering agents, forming the slurry into theapproximate desired shape, and then heating the formed slurry, alsocalled the green form, to evaporate the sintering agents and cause themetal particles to partially fuse. The sintered material is thenmechanically ground into the desired shape, such as the arc-shapedsegment 104 of FIG. 6. However, sintered material is not appropriate foruse in a high vacuum. The sintering agents are not completely drivenaway and the sintered material is relatively porous, making it almostimpossible to achieve the vacuum levels required in sputtering reactors.

[0049] The porosity problem is solved by encapsulating the poledmagnetic segments in a vacuum-tight annular carrier. As illustrated inthe cross-sectional view of FIG. 8, an annular moat-shaped structure 110is formed of a vacuum-compatible material such as SS304 stainless steel,a material which is non-magnetic, non-porous, vacuum-compatible, andweldable. Its thickness may be approximately 1.2 mm. The structure 110includes two sidewalls to be aligned with the central axis of thechamber and a perpendicular bottom wall, all of which are vacuum tight.Recesses 112 are formed at the top of the sidewalls. The structure alsoincludes at least three support tabs 114 of horizontal dimensions of afew millimeters extending radially outwardly from the outer sidewall toengage corresponding structure in the cover ring, as will be describedlater. More tabs 114 may be needed to support the very thin carrierstructure holding heavy magnets.

[0050] After all the magnet segments 104 have been poled in theirrespective directions, they are placed within the moat-shaped structure110 with their magnetization directions aligned as illustrated in FIG.5. Poling is typically accomplished by placing the magnet segments in avery strong magnetic field, possibly at a temperature just below theCurie temperature, with the segment being oriented such that the desiredmagnetization direction is parallel to the applied magnetic field. Asillustrated in the cross-sectional view of FIG. 8, an annular cover 116of the same stainless steel material is placed into the recesses 112 ofthe moat-shaped structure 110 to cover the moat. Two seams 118 betweenthe cover 116 and the moat sidewalls are vacuum sealed by localizedlaser welding to form the completed dipole ring 102 of stainless steelcarrier holding the magnets 104 without raising the magnets 104 abovetheir Curie temperature. Thereby, the porous magnetic material isencapsulated within the stainless steel carrier.

[0051] A magnetic dipole ring 102 has been fabricated according to theabove construction with 16 magnet segments 104 having a cross section ofabout 12 mm square. It produces a fairly uniform magnetic field of about70 gauss, which is considered sufficient for oriented magneticsputtering. A field of at least 25 gauss is desired. The small magnetwidth allows the dipole ring 102 to be placed between the bottom shield60 and the pedestal 72. The magnet height is sufficient to reliablyplace the wafer near the vertical middle of the magnets 104. To maximizemagnetic uniformity without interfering with sputtering dynamics, thewafer surface should be located within the upper half of the magnets 102with all or most of the magnetic region below the wafer surface.

[0052] The carrier may be formed in other ways. Rectangular magnetsegments can be accommodated in a polygonal rather than circularcarrier. Transversely magnetized cylindrical magnets may be received ina carrier having corresponding openings and means to angularly fix themagnets with the magnetization direction properly aligned.

[0053] The dipole ring 102 is conveniently mounted to the cover ring 100illustrated schematically in cross section in FIG. 4 but more accuratelyin the orthographic view of FIG. 9. The cover ring 100, preferablyformed of non-magnetic SS304 stainless steel, includes an annularinwardly extending top wall 120 and an annular, downwardly projectingouter sidewall 122 enclosing a central aperture 124. The cover ring 100is intended to protect the portions of the pedestal 72 radially outsideof the wafer 70 from sputter depositions. The wafer 70 fits within thecentral aperture 124, but unillustrated lift rings are used to lift thecover ring 100 off the pedestal 72 during wafer transfer. As shown inthe exploded cross-sectional view of FIG. 10, the top wall 120 of thecover ring 100 includes an annular support surface 126 for resting onthe periphery of the pedestal 72 or on an intermediate unillustrateddeposition ring. The top wall 120 also includes on its innermost bottomside a recess 128 so that the inward tip of the cover ring 100 does notrest on the pedestal 72 and risk being plated at that point to thepedestal 72. A middle centering ring 130 projects downwardly from thetop wall 120 and has a sloping inner side to center the cover ring 100on the periphery of the pedestal 72.

[0054] The outer sidewall 122 projects downwardly from the outer end ofthe top wall 120 and is typically used to overlap with the bottom shield60. For use with the dipole ring 102, an enlarged annular vault 134 isformed below the top wall 120 and between the sidewall 122 and thecentering ring 132 and is made large enough to accommodate the dipolering 102. Additionally, as illustrated orthographically in FIG. 9 and inmore detail in the exploded side view of FIG. 11, three or moreinterlocking cutouts or slots 140 arranged in hook-shaped structures areformed along the bottom edge of the cover sidewall 122 to accept andlock the tabs 114 formed in the carrier of the dipole ring 102. Eachcutout 140 includes a downwardly facing open end 142 wide enough to passthe width of the tab 114, a bottomed capture portion 144 also wideenough to accept the tab 114. An intermediate ridge 146 defines apassage between the open end 142 and the capture portion 144 of thecutout 140 that is tall enough to pass the thickness of the tab 114.

[0055] In assembly, all the tabs 114 of the dipole ring 102 are raisedinto the open ends 142 of the respective cutouts 140, the ring 102 isrotated to pass the tabs 114 over the ridges 142, and the ring 102 isthen dropped so that the tabs 114 are gravitationally captured in thecapture locking portions 144. Thereby, the dipole ring 102 isinterlocked and fixed within the vault 134 of the cover ring 100 whenthe assembly is lowered into the vacuum chamber during installation aslong as the cover ring 100 remains in its normal operationalorientation. The cover ring 100 is considered as a consumable item sinceit needs to be periodically replaced or refurbished when it becomesexcessively coated with sputter deposition. On the other hand, thedipole ring 102 is protected from being coated and should last muchlonger. The above design allows the dipole ring 102 to be easily removedfrom the cover ring 100 and then remounted on a fresh cover ring whenthe cover ring 100 is periodically replaced

[0056] The described cover ring protects only the pedestal. Othersimilarly configured rings, which will also be called cover rings, mayperform additional functions. They may additionally shield the peripheryof the wafer. They may clamp the wafer to the pedestal. They may focusthe plasma around the wafer. The integration of the dipole ring 102 withthe cover ring 100 provides several major advantages. The dipole ring102 is placed close to the wafer and can be made to occupy relativelysmall area in the confined plasma chamber. The chamber design other thanthe two rings 100, 102 and the composition of the target closely followsthe design for a non-magnetic sputter reactor, the latter of which willcontinue to dominate the commercial reactor market. Nonetheless, with amagnetic target, a magnetic dipole ring, and a redesigned cover ring,the widely available non-magnetic sputter reactor becomes a magneticsputter reactor. There is no need for a massive redesign of the chamberto accommodate auxiliary magnets.

[0057] When the magnetic dipole ring 102 is used to control theorientation of the ferromagnetic film on the wafer 70 supported on thepedestal 72, the magnetic orientation of the magnetic dipole ring 102needs to be maintained relative to the desired magnetization directionon the wafer 70. Variations of more than a few degrees of the film'smagnetic orientation, for example, more than 5 or 10°, willsignificantly degrade the operational characteristics of the fabricateddevices. The angular orientations of several elements need to becontrolled if the magnetic orientation is to be controlled. The wafertransfer mechanism responsible for placing the wafer 70 on the pedestal72 must accurately place the wafer 70 on the pedestal with correctangular orientation. The wafer orientation is indexed to an edge notchor optical indicia formed in the wafer 70. Conventional equipment isavailable for detecting the orientation inside a vacuum transfer chamberand accordingly reorienting the wafer.

[0058] Maintaining the orientation of the magnetic dipole ring 102involves at least three factors. The magnet segments 104 need to beindexed to the carrier of FIG. 8. This may be satisfied by carefulassembly, but a single transverse barrier in the moat of the carrierreferenced to a particular one of the tabs 114 eases this assemblyrequirements. The carrier of the dipole ring 102 needs to be angularlyaligned to the cover ring 100. This may be accomplished by using anirregular pattern of tabs 114 and cutouts 140 so that it is possible toassemble the dipole ring 102 and cover ring 100 in only one relativeangular orientation. Different mechanisms are possible to align thecover ring 100 to the pedestal and to maintain this alignment duringrepetitive operation. For example, the lift pins are located atparticular positions on the pedestal. The engagement of the lift pinswith the bottom of the cover ring 100 may include self-centeringindentations which not only provide the initial alignment but alsoprevent the cover ring 100 from walking around the circumference of thepedestal 72.

[0059] In practice, the magnetic uniformity produced by the magneticdipole ring could be improved. The number of magnet segments can beincreased to 16 or even 32. Further increases make the structure undulycomplex to manufacture. Even with 32 segments, non-uniform edge effectsare prominent. The more uniform region extends in an oval between thetwo magnet segments aligned with chamber radii. Sekine et al. and Miyatain the above cited patents attempt to improve the magnetic uniformity bychanging the form of the magnets and their arrangement around thechamber. However a simpler approach is based on the non-uniformity beingan edge effect. Accordingly, uniformity is improved by moving themagnetic dipole ring closer to the chamber sidewalls though preferablystill within the vacuum chamber.

[0060] In an alternative embodiment based on this effect and illustratedin the cross-sectional view of FIG. 12, a more conventional cover ring150 is supported during sputtering on a deposition ring 152, which restsin an annular recess at the periphery of the pedestal 72. An upwardlyextending inner wall 154 of the bottom shield 60 fits within adownwardly facing vault 156 of the cover ring 150. For loading andunloading of wafers 70, the pedestal 72 is lowered so that a terminatingupper end of the inner wall 154 of the bottom shield 60 supports thecover ring 150 and lifts it off the deposition ring 152. As the pedestal72 if further lowered, unillustrated lift pins in the pedestal 72 liftthe wafer 70 off the pedestal 72 while the deposition ring 152 continuesto rest on the pedestal 72. A wafer transfer paddle removes the wafer 70from the lift pins and places a new one there.

[0061] In this embodiment, an encapsulated dipole magnet ring 158 isdisposed between the chamber wall 42 and an downwardly extending outerwall 160 of the bottom shield 60. The dipole ring 158 may freely restupon centering dowels 162 extending from the chamber wall 42.Alternatively, the dipole ring 158 may be supported on the outer shieldwall 160 by a configuration that is inverse to the carrier tabs 114 andhook-shaped slots 140 in the cover ring 100 of FIGS. 7-11. For example,hook-shaped slots are formed in upwardly extending hangers formed in themagnet carrier near its upper inner corner of the carrier to engage tabsformed in the outer shield wall 160 and extending radially outwardlyfrom it.

[0062] The configuration of FIG. 12 however places a constraint on theplacement of the bottom shield 60 with respect to the chamber wall 42.To alleviate this constraint, in another embodiment illustrated in thecross-sectional view of FIG. 13, a dog leg is formed at the cornerbetween the outer wall 160 and a lower wall 164 of the bottom shield 60.The dog leg is formed by an annular ledge 166 and an annular middle wall168. The dipole ring 158 is fitted either partially or completely withina recess formed by the ledge 166 and the middle wall 168. It may beattached to either the ledge 166 or the middle wall 168 prior to theshield 60 being lowered into the vacuum chamber and detachably fixedthere by means similar to those described before or by other mechanicalfixing means. In either embodiment of FIGS. 12 and 13, the angularalignment of the dipole ring 158 with respect to the chamber 42 must bemaintained in order to align the magnetic material on the wafer 70 inthe correct direction.

[0063] A possible disadvantage of placing the magnetic dipole ringoutside the shield is that magnetic material is coated onto the shieldand could possibly shunt part of the magnetic field of the dipole ringand thus decrease magnetic intensity adjacent the wafer. However, thisis not considered a problem. The shields are typically replaced after10,000 cycles. MRAM applications typically require the deposition of nomore than 2 nm of magnetic material per cycle. Assuming full depositionon the shield, at the end of life the magnetic coating on the shield isno more than 20 μm, which is insufficient to screen the magnetic fieldfrom the interior of the shields.

[0064] Other types of attachments between the dipole ring carrier andeither the cover ring or the shield. The attachment may be made outsidethe chamber so threaded fasteners may be used. The attached assembly isthen lowered into the chamber.

[0065] An alternative embodiment of a magnet providing a uniformmagnetic field at the surface of the wafer 70 is illustrated in thecross sectional view of FIG. 14 and the plan view of FIG. 15. Auniformly horizontally magnetized layer 170 of magnetization M is fixedto the top of the pedestal 72 and supports the wafer 70 duringsputtering of the ferromagnetic material, which is deposited with thesame magnetization direction as the magnetization M of the magnetizedlayer 170.

[0066] A process has been developed for sputtering NiFe using theequipment of FIGS. 4-11. The use of the SIP magnetron and the groundeddark space shield promotes plasma ignition and stability. A plasma canbe struck and sustained at pressures as low as 0.8 milliTorr. Typicalpressure ranges for sputtering NiFe extend from 0.8 to 20 milliTorr.With the long-throw configuration and collimator, step coverage into0.25 μm holes with aspect ratios of 2 can be larger than 20%. Although a12 kW DC power supply was used in development, process powers formagnetic sputtering are typically in the range of 500 W to 6 kW for a200 mm wafer. At these powers, the sputtering ionization fraction isestimated to be less than 2%. As a result, the magnetic field at thewafer does not influence the dominant neutral sputter fraction.

[0067] Although the magnetic dipole ring has been developed for sputterdeposition of magnetically oriented coatings, it can also be used fordeposition of similar materials by chemical vapor deposition (CVD) fromone or more precursor gases that react to form a magnetic material onthe wafer. Preferably, however, thermal CVD is preferred becauseplasma-enhanced CVD tends to create intense plasmas and strong electricfields adjacent the wafer. An additional impressed horizontal magneticfield introduces significant asymmetries in plasma-enhanced CVD. Ofcourse, for thermal CVD with the dipole ring disposed inside thechamber, the magnets' Curie temperature needs to be higher than the CVDtemperature.

[0068] The invention has been described with respect to sputtering NiFe.Other ferromagnetic materials benefit from the invention. Theanti-ferromagnetic layer described above may be composed of MnFe orother anti-ferromagnetic metals and may be poled by being sputter coatedin the presence of the magnets described above in order to align itsdomains in the optimum direction. It is possible to sputter the freemagnetic layer in the presence of a horizontal magnetic field since itcan thereafter be selectively repolarized. Further, the magnetic dipolering can be used for more general purposes than permanent alignment offerromagnetic coatings. The characteristics of magnetizable materialscan be affected by the magnetic fields they experience during depositioneven though they may be deposited with no net magnetic polarity, forexample, by forming the easy magnetization direction in a specifieddirection. For example, the polarized deposition of the free magneticlayer may be used to orient any small magnetic domains along thedirection used for selective polarization. The polarized deposition ofthe free magnetic layer has the further advantage of allowing the samesputter reactor with included magnetic dipole ring to be used to depositboth the free and the fixed magnetic layers.

[0069] Although the invention has been described for arc-shaped magnetsegments which closely fit together, the magnets may have otherconfigurations including cylindrical magnets magnetized across theirdiameters such that a rotation of the cylindrical magnet in amulti-magnet annular carrier determines its magnetization direction.

[0070] Although sixteen magnet segments provide adequate fielduniformity, the uniformity can be improved by increasing the number ofmagnets, for example, to thirty two or even more. It may be possible todifferentially magnetize a unitary magnetic ring with the same magneticprecession. A magnetic dipole ring should have at least eight magnets toachieve any reasonable uniformity.

[0071] The sputter reactor described above employs both a collimator andlong throw to both isolate the horizontal aligning magnetic field fromthe plasma and magnetic field associated with the magnetron and topromote deep hole filling. Somewhat similar effects can be obtained byusing an even long throw ratio, for example, greater than 2 or 2.5,without the use of the collimator.

[0072] Accordingly, the various embodiments of the invention allow thedeposition of preferentially aligned magnetic materials with only minorchanges to a convention magnetron sputter reactor.

1. An annular cover ring for use in a plasma deposition reactor having apedestal for supporting on an upper horizontally extending supportsurface thereof a substrate to be coated and a magnetic ring disposedaround said pedestal, comprising: an annular roof portion covering aperiphery of said pedestal and extending radially outwardly from saidpedestal; an annular first projection extending downwardly from saidroof portion along a side of said pedestal; an annular second projectionextending downward from roof portion radially outside of said firstprojection, a downwardly facing vault formed by said roof portion andsaid first and second projections to accommodate therein said magneticring; and at least three attachments formed in said second projectiondetachably mounting said magnetic ring within said vault.
 2. The coverring of claim 1, wherein said magnetic ring comprises a magnetic dipolering creating a substantially uniform horizontal magnetic field on saidsupport surface.
 3. The cover ring of claim 2, wherein said dipole ringcomprises a plurality of permanent magnets arranged in a circle andhaving respective magnetic polarities precessing 720° around saidcircle.
 4. The cover ring of claim 1, wherein said first projection hasa sloped sidewall centering said cover ring on said pedestal.
 5. Thecover ring of claim 1, wherein said at least three attachments eachcomprise a hook-shaped slot formed at a bottom of said second projectionremovably receiving horizontally extending tabs on said magnetic ring.6. A sputter shield for use in a plasma sputter reactor having a centralaxis and placeable within and supportable on sidewalls of said reactor,comprising: an annular outer wall having at least an annular first wallportion extending along said central axis and including attachments fordetachably engaging a ring assembly fittable between said outer wall andsaid sidewalls; an annular bottom wall connected to said outer wall andextending perpendicular to said central axis; and an annular inner wallconnected to said bottom wall, disposed within said first portion ofsaid outer wall, extending along said central axis, and having aterminating end.
 7. A sputter shield assembly of claim 6, furthercomprising a magnetic dipole ring which is said ring assembly.
 8. Thesputter shield assembly of claim 6, wherein said magnetic dipole ringcomprises a plurality of permanent magnets arranged in a circle andhaving respective magnetic polarities precessing 720° around saidcircle.